ocellus

A simple light-receptor or primitive eye found
in many invertebrates. An ocellus typically
consists of a small number of sensory cells and a lens.
Although capable of sensing light, ocelli supply no information about the
direction from which it is coming.

Insects
have two types of ocelli. Dorsal ocelli occur in insects that also have
compound eyes. There are generally
three of them, forming a triangle on the dorsal surface or the top of the
head. Dorsal ocelli consist of a few tens of rod cells covered with a corneal
lens.

Lateral ocelli are the only eyes of the larvae of several orders of insects,
including fleas, silverfish, springtails, and Strepsiptera. They have a
mixture of rod cells and cone cells and are found on the sides of the head,
one to six on each side.

The term ocellus is also applied to eye-like markings, for example on some
butterfly wings or the long display feathers of peacocks. In many species,
such markings give protection by appearing threatening or by diverting attention
from vital body parts.